One critical issue confronting the fuel market is the introduction of Ultra Low Sulfur (ULS) fuels. The processes used to diminish sulfur content of fuels also impact other fuel properties. Fuel properties which are directly impacted by changes in fuel composition are fuel Electrostatics and fuel Lubricity.
The lubricity characteristic of a fuel affects engine and engine component durability, whereas the electrostatic characteristics affect risks associated with Static Discharge Ignitions (SDI). While the effect on mechanical durability by a fuel is an important consideration, the effect on safety of personal handling a fuel with increased probability of SDI is paramount.
The risks associated with SDI are well documented. In the 1980's and 1990's the American Petroleum Institute (API) compiled reports of road tanker explosions in Europe following the introduction of Low Sulfur Diesel (LSD), despite the use of grounding leads. These incidents were specifically attributed to static charge induced ignition of fuel vapor during fuel transfer operations.
Electrostatics:
It is widely known that electrostatic charges can be frictionally transferred between two dissimilar, nonconductive materials. When this occurs, the electrostatic charge thus created appears at the surfaces of the contacting materials. The magnitude of the generated charge is dependent upon the nature of and, more particularly, the respective conductivity of each material. The potential for electrostatic ignition and explosion is probably at its greatest during product handling, transfer, and transportation.
Electrostatic charging is known to occur during solvent or fuel pumping operations. In such operations, the flow of low conductivity liquid through conduits with high surface area or through “fine” filters combined with the disintegration of a liquid column and splashing during high speed tank loading can result in static charging. Such static charging can result in electrical discharge (spark) with catastrophic potential in highly flammable environments.
Thus, situations which are of greatest interest to the petroleum industry are conditions where charge builds up in or around flammable liquids and the possibility of discharge leading to incendiary sparking, and perhaps to a serious fire or explosion.
Countermeasures designed to prevent accumulation of electrostatic charges on a container being filled such as container grounding (i.e., “earthing”) and bonding are routinely employed. However, it has been recognized that grounding and bonding alone are insufficient to prevent electrostatic build-up in low conductivity organic liquids.
Organic liquids such as distillate fuels (diesel, gasoline, jet fuel, turbine fuels, home heating fuels, and kerosene), and relatively contaminant free light hydrocarbon oils (organic solvents and cleaning fluids) are inherently poor conductors. Static charge accumulates in these fluids because electric charge moves very slowly through these liquids and can take a considerable time to reach a surface which is grounded. Until the charge is dissipated, a high surface-voltage potential can be achieved which can create an incendiary spark, resulting in an ignition or an explosion.
The risk of static discharge ignition is further compounded by the newly enacted legislation designed to improve emissions characteristics from combustion of fossil fuels.
In order to meet emissions and fuel efficiency goals, automotive Original Equipment Manufacturers (OEM's) are investigating the use of NOx traps, particulate traps and direct injection technologies. Such traps and catalyst systems tend to be intolerant to sulfur. This coupled with the demonstrated adverse environmental consequences of burning sulfur rich fuels has resulted in a global effort to reduce the sulfur content of fuels (Reference World-Wide Fuel Charter, April 2000, Issued by ACEA, Alliance of Automobile Manufacturers, the entire teaching of which is incorporated herein by reference). These low sulfur and ultra-low sulfur fuels are becoming increasingly necessary to ensure compliance with emissions requirements over the full useful life of the latest technological generation of vehicles. Governments are also introducing further legislation for the reduction in particulate matter and fuel emissions.
In the United States, the Environmental Protection Agency (EPA) regulations require the sulfur content of on road fuel to meet Ultra Low Sulfur specification, specifically less than 15 ppm by mass of sulfur in the finished fuel. Similar regulations are also in place globally.
The method most commonly utilized to reduce the sulfur content of fuels is described as hydro treating. Hydro treating is a process by which hydrogen, under pressure, in the presence of a catalyst, reacts with sulfur compounds in the fuel to form hydrogen sulfide gas and a hydrocarbon. However, hydro treating to reduce sulfur content results not only in the removal of sulfur from the fuel but also the removal of other polar compounds which normally increase the conductivity characteristics of the fuel.
Generally, a non-hydro treated fuel has conductivity in the range of about 10 to about 30 pS/m2, whereas, a hydro treated fuel (below 15 ppm limit) is normally below 1 pS/m2. Conductivity below <3 pS/m greatly increases the risk of catastrophic electrostatic ignition. (Kattenwinkel, H. D., Electrical Conductivity, will a minimum level be required for all low S fuels in the future, 2nd CEN/TC 19 Symposium Automotive Fuels, 2003. Walmsley, H. L. An assessment of electrostatic ignition risks and filling rules for loading road tankers with low sulfur diesel, Institute of Petroleum, November 2000; the entire teachings of which are incorporated herein by reference).
In order to correct the detrimental effects of hydro treating, refineries and fuel handlers are routinely utilizing Static Dissipaters/Conductivity Improvers. These additives when used properly minimize the risk of electrostatic ignition in hydrocarbon fuels and solvents.
There is a great wealth of knowledge and experience regarding the use of Static Dissipaters/Conductivity Improver additives (ASTM D-4865 Standard Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems, and API Recommended Practice 2003—Protection Against Ignition Arising Out Of Static, Lightening, and Spray Currents; the entire teachings of which are incorporated herein by reference). The diversity of additives which have been patented and utilized in the fuel industry exemplifies the importance of risk associated with ignition due to static discharge.
The EPA ULS regulation and hydro treating required to meet sulfur requirements also greatly impacts lubricity properties of the fuels.
Fuel lubricity is the ability of the fuel to prevent wear on contacting metal surfaces. Certain diesel engine designs rely on fuel as a lubricant for their internal moving components. The problem of poor lubricity in these fuels is likely to be exacerbated by future engine system developments aimed at further decreasing emissions. This will result in an increase in the fuel oil lubricity requirement relative to requirements for present engines. For example, the use of high pressure unit injectors will likely increase the need for better fuel oil lubricity. Fuel lubricity requirements can be achieved by the use of lubricity additives.
As a consequence of the refinery processes employed to reduce Diesel sulfur and aromatics content, the majority of ultra-low sulfur Diesel fuels marketed today will require treatment with additives to restore fuel lubricity and fuel electrical conductivity.
Many additive producers and additive users are combining lubricity and conductivity additives into multipurpose packages to address the low lubricity and low conductivity problems associated with ULS fuels. Ostensibly, these combination packages not only delivering the required additives, but also, should enable the additive user to eliminate the requirement of maintaining two separate additive addition pumps and storage containers—however, these combination packages fall short of the desired product.
The pump systems utilized by the fuel industry to deliver additives into fuels have great difficulty in accurately delivering these additives at such low treatment rates. Therefore, fuel handlers using a single additive (conductivity) package are commonly required to make dilutions of the additive (usually with hydrocarbon solvents or fuels) prior to injecting the conductivity additive into the fuel.
In the combined packages, the dilution of the conductivity additive is achieved by utilizing the lubricity additive as the diluent. The lubricity additive can be used as a diluent because generally the amount of lubricity additive required to treat a fuel is generally 50 to 100 times the amount of conductivity additive required to treat the same fuel.
The regular practice in the oil refining and fuel additives industries of combining two or more additives to provide a multipurpose package comes with certain precautions and requirements.
It is, therefore, critical (especially with great safety concerns attributed to low conductivity) that the additives present in the fuel not only correct the specific fuel problems, or enhance the desired fuel attribute for which their addition is intended, but also, that the additives do not have a detrimental effect on other fuel properties, or on the performance of other additives present in the fuel. This requirement is commonly referred to as “No Harm”. The fuel and additives industries have developed a wide range of tests to evaluate the “no-harm” performance of additive packages and components. An example of such a testing protocol is ASTM D-4054, Evaluating the Compatibility of Additives with Aviation-Turbine Fuels and Aircraft Fuel System Materials, the entire teaching of which is incorporated herein by reference.
The present invention addresses the deficiencies of the prior art and the current and future requirements (Lubricity and Conductivity) associated with modern fuels.